1. Field of the Invention
Aspects of the present invention relate to a composite anode active material, an anode including the same and a lithium battery using the anode. More particularly, aspects of the present invention relate to a composite anode active material that has a large capacity and improved initial efficiency and cycle life performance by increasing the conductivity of the composite anode active material, an anode including the same and a lithium battery using the anode.
2. Description of the Related Art
Lithium metal has been used as an anode active material of conventional lithium batteries. However, when lithium metal is used, battery short-circuits may occur due to the formation of dendrites, which may cause a risk of explosion. Accordingly, carbon-based materials are widely used for the anode active material instead of lithium metal.
Examples of carbon-based active materials include crystalline-based carbon such as natural graphite and artificial graphite, and amorphous-based carbon such as soft carbon and hard carbon. Although amorphous-based carbon has high capacity, charge/discharge reactions are highly irreversible. Natural graphite is commonly used as main crystalline-based carbon, and the theoretical capacity thereof is high (372 mAh/g). Therefore, crystalline-based carbon is widely used as an anode active material. Such graphite or the carbon-based active material has a high theoretical capacity of 380 mAh/g. However, an anode including the above-mentioned anode active material is not desirable in lithium batteries that require a higher capacity.
In order to overcome this problem, research into metal-based anode active materials and intermetallic compound-based anode active materials has been actively conducted. For example, research on lithium batteries using metals or semimetals such as aluminum, germanium, silicon, tin, zinc, lead, or the like as anode active materials has been carried out. Such materials are known to have a large capacity, a high energy density, and a greater intercalation and deintercalation capability with respect to lithium ions compared to anode active materials using carbon-based materials. Thus, lithium batteries having a large capacity and a high energy density can be prepared using these materials. For example, pure silicon is known to have a high theoretical capacity of 4017 mAh/g.
However, such metal-based or intermetallic compound-based anode active materials have shorter cycle characteristics, that is, shorter cycle life performance than carbon-based materials, and thus have not yet been put to practical use. When an inorganic material such as silicon or tin is used in the anode active material as a lithium intercalating and deintercalating material, the volume of the inorganic material changes during a charge/discharge cycle, and thus, conductivity between the active materials may degrade or the anode active material may detach from the anode current collector. In particular, the volume of inorganic material such as silicon or tin increases by about 300 to 400% through intercalation of lithium during charging, and the volume thereof decreases through deintercalation of lithium during the discharging. Therefore, when charge/discharge cycles are repeated, spaces may be generated between the inorganic particles and active materials, electrical insulation or reduced conductivity may occur, and the cycle life performance of the battery may therefore be rapidly reduced. Thus, the inorganic material can not be used in lithium batteries.
In order to overcome this problem, Japanese Patent Laid-Open No. 1994-318454 discloses an anode manufactured by simply mixing a carbon-based active material in which lithium can be intercalated and deintercalated, with metal or alloy particles. However, in this case, the volume of a metal-based anode active material excessively expands and contracts during charge/discharge cycles, and thus the metal-based anode active material may be easily broken into small pieces. The broken particles detach from the current collector, and thus the cycle life performance of the anode rapidly deteriorates. Japanese Patent Laid-Open No. 2006-147316 discloses an anode having an active material layer formed in a current collector having pores formed on the surface thereof, wherein the active material layer contains active material particles made of metal, which has a higher capacity than graphite, and particles of a carbon-based material or rubber-like material. However, in this case, the particles of the rubber-like material function as a binder by being simply mixed in an electrode, and thus only serve to prevent the anode from being deformed during charge/discharge cycles. In addition, the manufacturing process is complex, and capacity, efficiency and cycle life performance are insufficiently improved.